Electrostatic precipitators and applications
Key idea: An electrostatic precipitator charges dust particles so that oppositely charged plates attract them out of the smoke. It removes particles, not polluting gases.
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Guided steps
- Read ion transfer, electric collection and deposit removal in the actual figure.
- Follow all three physical stages and the pollutant limit.
- Explain charging, collection and removal, then predict failed charging.
- Choose the missing first stage, then explain the complete process.
- Write a complete replacement explanation with charging and removal.
- Explain how the earthed plate attracts positive dust.
- Find what is charged and what attracts it in each device.
- Explain droplet repulsion and attraction to the earthed car.
- Explain the charge pattern, toner transfer and fixing.
- Apply droplet repulsion and Earth attraction to crop spraying.
- Calculate dust collection and decide whether carbon dioxide is removed.
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Static charge causes sparks and shocks, but it can also be put to work. Every useful device in this lesson does the same two things: it charges the particles it wants to move, then uses an attraction to send them where they are wanted.
The electrostatic precipitator
Burning coal or oil in a power station or factory makes smoke full of fine ash and soot. An electrostatic precipitator removes these particles from the waste gas before it leaves the chimney.
- Charge the dust. The smoky gas flows past metal wires held at a very high negative voltage. A discharge in the strong field near the wires produces ions. Negative ions attach to dust particles and transfer charge to them as they pass. The dust becomes negatively charged.
- Attract the dust to the plates. The gas then flows between large metal plates that are earthed. The negatively charged dust is attracted to the plates and sticks to them. Gas with fewer particles carries on up the chimney; collecting ash does not remove gaseous carbon dioxide or sulfur dioxide.
- Remove the dust. Every so often the plates are shaken or knocked, and the layer of dust falls into a hopper below to be taken away.
Worked example 1
Explaining a precipitator
Problem
Explain how an electrostatic precipitator removes ash particles from the waste gas of a power station.
Show full solution
Charge
Method
The ash particles are given a negative charge as they pass the high-voltage wires.
Reason
Charging gives ash particles a net electric force that moves them towards the collectors rather than simply following the gas flow.
Working
Answer
Ash → negatively charged ash.Attract
Method
The charged ash is attracted to the earthed plates and sticks to them.
Reason
The plates attract the negative particles, pulling them out of the gas flow.
Working
Answer
Charged ash → plates; gas with fewer particles → chimney.
Remove
Method
The plates are shaken and the ash falls into a hopper.
Reason
Accumulated deposits can reduce effective collection and must be removed so the device can keep operating.
Working
Answer
Plates shaken → ash collected below.
Quick check
Check your understanding 1 Follow the dust
Using the diagram, describe how dust gains charge, moves to the collectors and is removed. What happens to collection if the charging wires stop working?
Show answer
Negative ions from the high-voltage wires charge the dust. Electric attraction draws it towards the earthed collectors. Shaking the plates drops the deposits into a hopper. Without the charging stage, electric collection weakens and more dust leaves with the gas. Check that you connect charge, force and removal.
Check your understanding 2 What must happen before collection?
A description starts with plates attracting dust from smoke. Which essential stage should it describe first?
Show answer
The particles must first acquire charge. A complete explanation also follows their electric-force collection and the removal of accumulated deposits.
Spot the mistake 1
The plates do it all?
Learner answer
A student writes: “The charged plates attract the dust out of the smoke.” The answer scores 1 mark out of 3. What is missing?
Which step is missing?
Now write a complete replacement explanation, including how the dust acquires charge and what happens after it reaches the collectors.
Show solution step by step
The first step
Method
The answer does not say how the dust gets its charge.Reason
It is the charge on the dust that lets the plates pull it strongly out of the moving gas.
Working
Answer
The dust is charged as it passes the high-voltage wires.
The last step
Method
The answer stops at the plates.Reason
The dust has to be taken away, or the plates fill up.Working
Answer
The plates are shaken and the dust falls into a hopper. A complete account links ions charging the dust, electric force collecting it on the plates, and deposits being removed so collection can continue.
Common mistake
Check your understanding 3 Positive dust
A redesigned precipitator gives the dust a positive charge. Explain how an earthed metal plate can still attract it.
Show answer
Positive dust attracts electrons from Earth towards the near surface of the plate. The negative surface charge attracts the dust. Earthing lets electrons move between the plate and Earth; an earthed plate need not carry a fixed positive charge.
Using the same idea elsewhere
To explain any device that uses static charge, answer two questions: what is charged, and what attracts it to where it is wanted?
- Spray painting. Paint droplets are charged as they leave the spray gun, and a conducting object being painted is earthed. Electric attraction can increase deposition on it, including some deflection around edges.
- Photocopiers and laser printers. In a reflected-light photocopier, a photosensitive drum is charged and illumination makes charge leak away in regions corresponding to white parts of the original. Oppositely charged toner collects on the retained-charge image, transfers to paper and is fixed by heat. Laser printers also form a controlled charge pattern and transfer toner, but write it with laser light; the illuminated regions that retain toner depend on the printer design.
Guided practice 1
Why charged paint wastes less
Problem
Paint droplets all carry the same sign of charge. A conducting car body is earthed. Explain how electrostatic forces help the spray spread and make droplets land on the car.
What is charged, and what attracts it?
Hints
Hint 1
The droplets
Every droplet has the same sign of charge. What do they do to each other?
Hint 2
The car
What does the earthed car body do to the droplets?
Show solution step by step
The droplets spread out
Method
Like-charged droplets repel one another, helping the spray spread.
Reason
They all have the same sign of charge, and like charges repel.
Working
Answer
Like charges repel, helping the droplets spread.
The car attracts them
Method
The earthed car body attracts the droplets, increasing the amount of paint deposited on it.
Reason
Charged droplets are attracted to an earthed object.Working
Answer
More droplets land on the body, reducing paint waste.
Quick check
Earthing matters too. As droplets land, their charge flows to Earth, so the car never builds up a charge that would repel later droplets.
Exam-style question 1
How a photocopier makes an image
Examination question
In the reflected-light photocopier model described above, explain how static charge copies black text onto paper. Include how the drum becomes more conducting where illuminated and how toner moves from drum to paper. [4 marks]
Charge, light, toner, paper
Show solution step by step
Make a charge image
2 marksMethod
The drum is charged all over. Light reflected from the white parts of the page falls on the drum and lets the charge there leak away.
Reason
Black text reflects much less light than the white background. Here, the illuminated drum regions conduct and discharge, while the darker regions retain the charge pattern of the text.
Working
Answer
Charged drum → charge left where the text is.
Transfer the toner
2 marksMethod
Toner with the opposite charge is attracted to the charged parts of the drum. An electric transfer arrangement attracts the toner from the drum to the paper; heat and pressure then fix it in place.
Reason
Opposite charges attract, so toner is drawn to the retained charge pattern.
Working
Answer
Toner → drum → paper → heated.
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Give one mark for each point your answer states.
Common mistake
Saying the light adds charge to the drum loses the second mark: light reflected from the white parts removes the charge there.
Try it yourself 1
Spraying crops
Problem
Plants have a conducting path to Earth through moist roots and damp soil. Explain how charging insecticide droplets could help them spread and land on the leaves.
What do you charge, and what attracts it?
Hints
Hint 1
Which part is like spray painting?
The droplets play the part of the paint. What plays the part of the car?
Hint 2
Are the plants earthed?
Living plants are moist and rooted in damp soil.
Show solution step by step
Charge the droplets
Method
Give the insecticide droplets a charge as they leave the sprayer.
Reason
Like-charged droplets repel each other, helping the spray spread.
Working
Answer
Like-charged droplets repel.
Let the plants attract them
Method
The plants are connected to the earth through their roots and the damp soil, so they attract the charged droplets.
Reason
The earthed plants attract the charged droplets, helping more spray reach the leaves.
Working
Answer
Charged droplets are attracted towards the conducting, earthed plants.
Quick check
Check your understanding 4 Measure dust collection
A dry precipitator receives 80 mg of dust each second and emits 12 mg each second. Calculate the collected percentage. The gas also contains carbon dioxide. Does collecting the dust remove this gas?
Show the steps Optional working
Show answer
Collected dust = 80 − 12 = 68 mg each second. Collected percentage = 68/80 × 100% = 85%. The carbon dioxide remains in the gas, and 12 mg of dust still leaves each second. A clearer-looking outlet can still contain pollutants.
Common mistakes
- Leaving out the charging step. Name what is charged, and how, before saying what attracts it.
- Saying the plates “suck in” the dust. They attract charged particles.
- Stopping a precipitator answer at the plates. Say how the dust is removed from them.
- Saying a precipitator cleans gases such as carbon dioxide. This ash-collection process removes particles; it is not a method for removing gaseous carbon dioxide.
- Giving the target the same charge as the particles. It must be earthed or have the opposite charge.
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